enable cross-window tab dragging in preview

[peicodes]

This PR enables tab dragging in preview. There are two main issues:

1. Dragging a tab out and in repeatedly in the same mousedown doesn't work: investigation for tab-redragging #12044
2. @acarl005 reported a random crash on Windows. I tried using a VM to reproduce but have been unsuccessful.

#1 is not trivial to fix, and I'm deliberately scoping out of Preview. I'll take another pass before considering this feature for Stable.

I'm hoping to leverage Preview users to hone in on #2, but will continue to attempt to reproduce until the Preview release on Thursday.

[oz-for-oss]

@peicodes

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[oz-for-oss]

Overview

This PR promotes DragTabsToWindows from Dogfood to Preview by adding it to PREVIEW_FLAGS.

Concerns

• The PR description calls out an unreproduced random crash on Windows, but the new Preview flag entry enables the feature for Windows Preview users too.
• This is a user-facing behavior change, but the PR description does not include screenshots or a screen recording demonstrating cross-window tab dragging working end to end.

Verdict

Found: 0 critical, 2 important, 0 suggestions

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[oz-for-oss]

⚠️ [IMPORTANT] The PR description calls out an unreproduced Windows crash, but this promotes the flag to Preview for Windows users too. Gate the Preview entry on non-Windows until the crash is fixed or ruled out.

Suggested change

	pub const PREVIEW_FLAGS: &[FeatureFlag] = &[
	FeatureFlag::DragTabsToWindows,
	];
	pub const PREVIEW_FLAGS: &[FeatureFlag] = &[
	#[cfg(not(windows))]
	FeatureFlag::DragTabsToWindows,
	];